This invention generally relates to formation of an alignment mark, and more particularly relates to a method for forming an alignment mark which is employed in the process of manufacturing a silicon semiconductor device, especially in the process of forming a microscopic electronic structure on the surface of a semiconductor or silicon substrate. The invention also relates to a method for forming a structure covering the alignment mark for protection thereof.
In the process of manufacturing a semiconductor device, the photolithographic process is generally employed to process a desired microscopic semiconductor structure on a semiconductor or silicon substrate surface. This photolithographic process typically consists of the steps of forming a photoresist film to cover the silicon substrate surface; projecting the light on the photoresist film to focus a predetermined light pattern thereon with help of an optical projector; developing in part the photoresist film exposed to the light; removing the remaining photoresist film unexposed to the light; and etching the semiconductor substrate surface portion as revealed by removing the photoresist film until the desired pattern protected by the photoresist is appreciably obtained. This photolithographic process is repetitively performed using several different patterns until the desired microscopic semiconductor structure is completed on the semiconductor substrate surface. Therefore, in the photolithographic process, it is strictly required to accurately align the pattern previously formed on the substrate with a pattern which is to be formed next thereon. In other words, the photolithographic process requires a so-called precise alignment process for aligning one pattern with the next. In order to accurately carry out this process, some marks are employed as alignment marks which are provided in predetermined positions on the substrate surface, and relative positioning between patterns is performed referring to these marks.
Typically, formation of these marks may be simultaneously executed in parallel with other processes such as a process for forming an electrical wiring over the substrate. This process will now be described with reference to FIGS. 4 and 5 showing a prior art method in which formation of the alignment mark is carried out in parallel with formation of an aluminum wiring over the substrate. At first, a silicon oxide film layer 12 is formed to cover the surface of a silicon substrate 10. While some holes for use in the aluminum wiring are bored by etching the oxide film layer 12 with a suitable etchant, formation of recesses 14 is carried out by etching the oxide film 12 at predetermined positions where alignment marks are expected to be located. As shown in FIG. 5, after the above etching process of the oxide film layer 12, a conductive material such as aluminum (Al) is sputtered over the oxide film layer 12 to form an Al wiring layer 16. When the Al wiring layer 16 is completed, such a recess as indicated in the figure comes out on the surface of the Al wiring layer 16, in correspondence with the position of each recess 14 formed by etching the oxide film layer 12 in part. Thus, these recesses can be used as alignment marks AM. As described above, the recess 14 plays a role as a base which defines the position of a designed or planned alignment mark AM. Therefore, for the purpose of simplifying the description, the recess 14 will be referred to as xe2x80x98base recessxe2x80x99 hereinafter in this specification as well as in the recitation of claims as attached hereto.
In case of forming the base recess 14 for alignment mark AM by etching the oxide film layer 12 in part, its depth has to be carefully controlled, as shown in FIGS. 4 and 5, to avoid that the base recess 14 reaches the silicon substrate 10 by excess etching. Despite the control of this etching process, however, such excess etching might sometimes happen depending on unexpected change in the etching condition, for instance etching time, etching temperature, etchant, etc. Once such excess etching has happened, the base recess reaches the silicon substrate, so that the Al wiring layer 16 directly comes in contact with metallic silicon of the silicon substrate 10 when the Al wiring layer 16 is formed over the oxide film layer 12. Aluminum and silicon are essentially so active to each other that an Alxe2x80x94Si alloy is formed at the interface therebetween, if the Al wiring layer 16 is formed at a high temperature such as 500xc2x0 C. Therefore, according to the prior art method as described above, when aluminum wiring layer 16 and silicon substrate Si come in contact with each other under such high temperature condition, Alxe2x80x94Si chemical reaction rapidly and vigorously proceeds to produce a reaction layer 18, and the oxide film layer 12 is violently attacked and destroyed during this chemical reaction, as shown in FIG. 6, thereby the desired alignment mark AM being eliminated.
Accordingly, the invention has been made in view of such a problem as described above. Therefore, the first object of the invention is to provide a novel and improved method for forming alignment mark which can be stably maintained without being destroyed by rapid and vigorous Alxe2x80x94Si chemical reaction even when the alignment mark and the Al wiring layer are simultaneously formed at a high temperature.
Another object of the invention is to provide a novel and improved method for covering an alignment mark, according to which the alignment mark already formed on the silicon substrate can be protected from being destroyed by rapid and vigorous Alxe2x80x94Si chemical reaction.
In order to solve such problem as mentioned in the above, according to the first aspect of the invention, there is provided a method for forming an alignment mark, which includes the steps of forming an oxide film over the surface of a silicon substrate, forming a base recess by etching the oxide film in part, and forming an Al wiring layer over the oxide film so as to fill up the base recess therewith, thereby causing a mark as an alignment mark to come out at a point on the surface of the Al wiring layer in correspondence with the position of the base recess. In this case, according to the present invention, the base recess is formed such that it has a depth less than that (thickness) of the oxide film, so that the metallic silicon of the silicon substrate can not make any direct contact with the Al wiring layer.
As described above, since the alignment mark according to the invention can be formed without causing any direct contact between the Al wiring layer and the metallic silicon of the silicon substrate, there is no possibility that they chemically reacts with each other. Thus, there is caused neither deterioration in the quality of the oxide film, nor destruction of the alignment mark formed on the surface of the Al wiring layer.
To put it more concretely, it may be possible to achieve such an alignment mark as described in the above by using a method including the steps of additionally forming the second oxide film over the oxide film which is initially formed over the silicon substrate surface, and then forming the base recess by etching this second oxide film in part. With the addition of the second oxide film, there will be further reduced a possibility that the metallic silicon of the silicon substrate is exposed to the inside of the base recess, even though the depth of the base recess is made deeper to some extent, thus the Al wiring layer being prevented from making a direct contact with the metallic silicon of the silicon substrate.
Furthermore, according to the second aspect of the invention, an alignment mark may be obtained by using a method including the steps of forming an aluminum layer over the oxide film which is initially formed over the silicon substrate surface, then forming second oxide film over the aluminum layer, forming the base recess by etching the second oxide layer in part, and finally forming the Al wiring layer over the second oxide film. With addition of this aluminum layer, there is eliminated a possibility that the base recess for alignment mark reaches the metallic silicon of the silicon substrate, penetrating through the aluminum layer. Consequently, the metallic silicon of the silicon substrate is not exposed to the inside of the base recess and the Al wiring layer is prevented from making a direct contact with the metallic silicon of the silicon substrate.
Still further, according to the third aspect of the invention, there is provided a structure for covering the alignment mark, which is formed by a method including the steps of forming an oxide film over the surface of a silicon substrate, forming a base recess by etching the oxide film in part, and forming the second oxide film over the initial oxide film over the silicon substrate so as to fill up the base recess already formed therein. According to this covering structure, since the base recess formed in the oxide film over the silicon substrate is filled up with the second oxide, the Al wiring layer to be formed over the second oxide film can not come in contact with the metallic silicon of the silicon substrate. Consequently, there is caused neither Alxe2x80x94Si chemical reaction nor destruction of the base recess already formed in the oxide film over the silicon substrate.